This invention relates in general to two coordinate optical scanning systems and more particularly to a system and method for correcting cross scan error in such systems.
In the art of optical instruments, it is known to scan a surface to be imaged with a small cross section light beam, collect the light reflected from the illuminated spot and direct it to a detector which provides an output signal varying in time in correlation with the scanning of the illuminated spot across the surface. The detector output can be stored in a permanent storage medium or provided directly to a scanning display device, such as a television raster or a cathode ray tube display. By synchronizing the scanning operation of the illuminating source with the scanning of the display signals, a two dimensional image is produced.
There currently exist a number of systems for scanning a small beam of light or a laser beam over the two coordinates, usually orthogonal, to image the object being scanned. Two examples of instruments which employ such scanners are double scanning ophthalmoscopes and scanning microscopes. One design of such instruments utilizes a rotating multi-faceted polygon to generate the light beam scan along one coordinate, typically the horizontal coordinate, followed by a reflection galvanometer to produce a scan along the other coordinate. One source of error in these instruments arises from the lack of parallelism of the reflecting facets of the polygon. This difference in tilt of the facets along the coordinate orthogonal to the one being scanned produces runout error which appears to the instrument as an error in vertical scan. In single scanning systems this error has been corrected by means of a focusing element placed after the scanner which focuses the beam along this orthogonal coordinate at the surface of the object to be imaged, thus eliminating at the object the positional error otherwise introduced. However, in a two-coordinate scanning system this compensation cannot be made because to do so would remove the vertical scanning process itself.